Commercially available Optical Coherence Tomography (OCT) systems are employed in diverse applications, including diagnostic medicine such as ophthalmology, where they are used to obtain images of the retina.
In conventional interferometry with long coherence length (laser interferometry), interference of light occurs over long distances. In white light OCT, which is based on broadband light sources, the interference is shortened to a distance of micrometers owing to the short coherence lengths of the light sources (the coherence length is reciprocal to the bandwidth of the light source).
In dual path interferometry, the incoming light is split into two arms—a sample arm (containing the item of interest) and a reference arm (usually a mirror). The combination of the reflected light from the object in the sample arm and reference light reflected from the mirror in the reference arm gives rise to an interference pattern. In interferometry with short coherence length such as OCT, interference patterns are obtained only when the optical path difference (OPD) between the light from both arms is less than the coherence length of the light source.
Time Domain OCT (TD-OCT)
In time domain OCT, the mirror in the reference arm is progressed longitudinally in time. Since the fringes are obtained only when the OPD is shorter than the coherence length of the light source, the envelope of the visible fringes changes as the OPD varies, the peak of the envelope corresponding to zero OPD. This interference is called auto-correlation and the intensity of the interference as a function of the OPD is called an interferogram. By scanning the mirror in the reference arm and measuring the OPD where the peak of the envelope is obtained, the height profile of the sample can be obtained.
Frequency Domain OCT (FD-OCT)
In frequency domain OCT, the mirror in the reference arm is fixed and the broadband interference is acquired by measuring the spectrum of the reflected light. According to the Wiener-Khintchine theorem there is a Fourier relation between the interferogram and the spectral power density. The interferogram and thus the depth scan can be calculated by Fourier-transforming the measured spectrum of the reflected light. The FD-OCT improves the imaging speed, reduces losses during the scan and improves the signal to noise ratio compares to TD-OCT.
Swept Source OCT (SW-OCT)
In SW-OCT, the mirror in the reference arm is fixed but the spectral components are encoded in time. The spectrum is either filtered or generated in a series of successive frequency steps. The measured reflected light as a function of optical frequency is Fourier-transformed to obtain the interferogram.
Scanning
The systems described above are based on single point depth information obtained by the OCT; hence they scan the sample in two lateral dimensions and reconstruct a three-dimensional image of the object. The cross-sectional scan is called B-scan.
The FD-OCT and the SW-OCT have much higher signal to noise ratio (SNR) than the TD-OCT, but need expensive optical devices such as a high resolution spectrometer and a tunable laser.
Therefore there is a need for a new method for low-cost OCT which still has high SNR.